1. Field of the Invention
The present invention relates to a tripod-type constant velocity joint used in a force transmission system of a vehicle; for example, between each side gear of a differential gear unit and a drive axle for driving a wheel.
2. Description of the Related Art
As shown in, for example, Japanese Utility Model Application Laid-Open (kokai) No. H4-84923, a tripod-type constant velocity joint comprises a tripod case (outer joint member), a trunnion (inner joint member), and three joint rollers (roller units). The tripod case has three roller grooves (guide grooves) formed on the inner surface thereof and extending along the axial direction. The trunnion has three trunnion shafts (tripod shafts) each extending radially outward and having a peripheral surface in the form of a partial sphere (tripod spherical surface) at the distal end thereof. Each of the joint rollers has an inner circumferential surface (center hole) which is slidably fitted onto the corresponding partial spherical peripheral surface of the trunnion, and an outer circumferential surface that is engaged with the corresponding roller groove of the tripod case such that the joint roller can roll only along the axial direction. In the tripod-type constant velocity joint disclosed in the publication, each joint roller is composed of an outer race (outer roller), and an inner race (inner ring) that is rotatably engaged with the inner circumferential surface of the outer race via needle rollers (rolling elements). The needle rollers reduce frictional resistance between the trunnion shafts and the joint rollers, which roll within the corresponding roller grooves when the joint is rotated with a joint angle formed between the tripod case and the trunnion, and reduce thrust forces which press the tripod case and the trunnion toward axially opposite directions, to thereby prevent occurrence of problems of generation of vibration and noise and loss of power.
In such a tripod-type constant velocity joint, the inner joint member carrying the roller units has its largest radius at the outer-side corner portion of the outer circumference of each roller unit, and the inner surface of the outer joint member has its maximum radius at portions each facing the corresponding corner portion. Therefore, the maximum outer radius of the outer joint member; i.e., the maximum outer radius of the constant velocity joint, is equal to the sum of the maximum radius of the inner surface of the outer joint member and a wall thickness required in consideration of mechanical strength. In order to reduce the size of the joint through reduction of the maximum outer radius thereof, the corner portions of the outer circumference of each roller unit may be chamfered or tapered such that the width of the outer circumferential surface of each roller unit becomes smaller than the width of the roller unit (see FIG. 2). However, the above-described configuration for reducing the size of the joint causes the following problem, particularly in the case where each roller unit is composed of an outer roller and an inner ring and has a large diameter, although such a problem arises even in the case where each roller unit is a single member of a simple configuration.
Each guide groove of the outer joint member has a pair of elongated, parallel flat surfaces facing each other, and the outer circumferential surface of each roller unit is located between and in engagement with the elongated flat surfaces, so that the roller unit can roll only along the longitudinal direction of the guide groove, whereby movement of each roller unit in other directions with respect to the corresponding guide groove is restricted. Via such roller units, force is transmitted between the outer joint member and the inner joint member of the constant velocity joint. Specifically, depending on the direction of transmission of force, one of the elongated flat surfaces comes into contact with the outer circumferential surface of the corresponding roller unit, whereby force is transmitted from the outer joint member to the corresponding tripod spherical surface of the inner joint member via the roller unit, and a slight clearance is produced between the other elongated flat surface and the outer circumferential surface of the roller unit.
As described above, each roller unit is allowed to roll only along the longitudinal direction of the corresponding guide groove, and is restricted from moving in other directions with respect to the corresponding guide groove. Therefore, each roller does not move with respect to the outer joint member in a direction perpendicular to the center axis thereof. In contrast, when the constant velocity joint is rotated with a joint angle formed, each tripod spherical surface of the inner joint member moves with respect to the outer joint member in a direction perpendicular to the center axis thereof. Therefore, each tripod spherical surface fitted in the cylindrical center hole of the corresponding roller unit slides within the center hole. Because of this sliding movement, a frictional force is generated along the wall surface of the center hole at the position of contact between the tripod spherical surface and the wall surface of the center hole, wherein force is transmitted through the contact point. Thus, the line of action of the force transmitted from the wall surface of the center hole to the tripod spherical surface via the contact point inclines from a direction perpendicular to the center axis of the corresponding tripod shaft extending radially.
In the case where the width of the outer circumferential surface of each roller unit is made smaller than the width of the roller unit as described above, the inclined line of action of the force deviates from the outer circumferential surface of the roller unit in contact with one of the elongated flat surfaces of the guide groove, and at the opposite side, the outer circumferential surface of the roller unit separates from the other elongated flat surface, with the possible result that the roller unit inclines. When such an inclination is produced, on the side opposite the side on which force is transmitted, the roller unit comes into contact with the inner surface of the guide groove, and because of the friction resistance produced therebetween, there is produced a thrust force which presses the outer and inner joint members toward axially opposite directions. Since this thrust force abruptly changes with rotational angle of the constant velocity joint, the problem of generation of vibration and noise and loss of power arises.